The Night Sky This Month: September 2016

In September, the “Summer Triangle” continues to be a regular feature of the night sky with Vega in the constellation Lyra, Deneb in Cygnus, and Altair in Aquila observed overhead soon after night fall. Another beautiful asterism on display this month is the Great Square of Pegasus, which forms the winged horse’s body, with its top left star then branching off to become part of the constellation Andromeda. Other constellations bordering Andromeda includes Perseus, Cassiopeia, Pisces and Triangulum.

The Moon & Phases

Favourable illumination on the 9th, and again on the 22nd of the month offers lunar observers two excellent opportunities to view the Alpine Valley, a dramatic cleft through the Appenine Mountains to the east of the crater Plato. At 7 miles wide and 79 miles long, the Alpine Valley is an easy target even for modest telescopes when conditions allow such as when the terminator is close by, as it will be on the aforementioned dates. In case you were wondering, the terminator refers to “the dividing line between the illuminated and shadowed portions of the lunar or planetary disk.”

Mercury will reach superior conjunction on September 12th, but by the last week of September, it will reappear as a pre-dawn object. During this time, its brightness will increase by about three full magnitudes from +2.5 on the 19th to a blazing -0.4 on the 28th. On the latter date the little planet will be about 18 degrees from the Sun (greatest elongation), and will rise about 90 minutes before dawn, when it will be visible about 9 degrees above the eastern horizon.

Venus starts the month barely a few degrees above the western horizon, when it sets about 60 minutes or so after the Sun. However, it sets progressively later as the month wears on, but only by about 15 minutes by month’s end, which makes September not a good month to observe the planet.

Mars is now moving towards the east, and away from Saturn and Antares, with which it was in a close grouping during the last part of August. However, it is still visible low on the south-south-western horizon just after sunset, but its brightness diminishes from magnitude -0.3 to +0.1, while it’s angular diameter shrinks from 10.5 to 8.8 seconds of arc as the month progresses. On the 19th of the month, the Red planet will be almost exactly one Astronomical Unit away from the Earth, and two days later, on the 21st, it leaves Ophiuchus and passes into Sagittarius. Given its low elevation though, it is unlikely that any but the biggest telescopes will reveal any surface details and features on the planet.

Jupiter is only visible for the first week or so of September since it is moving towards a position of superior conjunction with the Sun, which it reaches on the 26th of the month. However, given its magnitude of -1.7 and angular diameter of 31 seconds of arc at the start of the month, it might just still be possible to observe the King of the planets’ equatorial bands and four Galilean moons before the planet becomes lost in the Sun’s glare. The better option might be to wait until October for Jupiter to reappear again the pre-dawn sky.

Saturn starts the month only about 10 degrees above the south-western horizon in the southern reaches of the constellation Ophiuchus. The planet is also approaching the Sun, meaning that despite its current magnitude of +0.5, and angular diameter of 16.7 (reducing to 15.9) seconds of arc, the first week or so of the month is the last opportunity to view the ring system that is still tilted towards our line of sight by 26 degrees.

Neptune reaches its point of closest approach (opposition) to Earth on the 2nd of the month, when it will have an angular diameter of 2.3 seconds of arc, and shine at magnitude +7.8, which makes it an easy target for small telescopes. As the month wears on, Neptune will reach a point about 27 degrees above the southern horizon, so it should be possible to spot the planet’s largest moon, Triton with telescopes of 8-inch aperture and bigger if seeing conditions allow.

Meteor Showers

There are no major meteors showers during September, although it might be worth staying up late for two minor showers that are visible from the UK during the month. Below are some details:

While it is generally true that planetary nebulae don’t look like much in amateur telescopes, M57 in Lyra is an exception. It is one of only a few nebulae that show something of its true colour when viewed directly, but note that only the green portion of its radiated light is visible in a small instrument, mostly because human vision is more sensitive to the colour green than any other colour. The remains of the progenitor star that is shown here, as the bright spot in the centre of the structure, is also not visible in amateur equipment.

Albireo

The double star that marks out the “head” of Cygnus, the Swan, is arguably the most beautiful double star in the entire sky. While there are many other known examples of starkly contrasting double stars visible in small telescopes in terms of the colours of the components, there is no other binary star known that contrasts as sharply as this pair: the image below shows the components at the correct relative scale, both in terms of size and colour.

Brocchi’s Cluster

Also in Cygnus, and popularly known as “The Coat Hanger” this asterism is an easy binocular target just to the lower left of the double star Albireo. To the casual observer it might appear as if the Coat Hanger is located in an empty region of the sky, since there are few stars around it, but this is only an effect of the dark dust lane behind the asterism that obscures the light from surrounding stars.

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